FOLLOW-UP: Passivation Before Heat Treatment

I received a very good email from D.H. discussing the August 2007 question regarding the passivation and apparent rust on 17-4PH precipitation hardenable stainless steel.


To Readers: I received a very good email from D.H. discussing the August 2007 question regarding the passivation and apparent rust on 17-4PH precipitation hardenable stainless steel. One significant point that I did not mention was that the passivation process may be better done prior to heat treatment. Either that, or an acid cleaning process may be necessary to remove surface contaminants that could be diffused into the surface depending on the time, temperature and atmosphere of the heat treatment.

However, I still think it may be necessary to provide a passivation of the surface following the heat treatment, even if it is done prior. If the atmosphere in the furnace is slightly reducing (to avoid darkening or oxidation of the metal surface), the passivation treatment will only be partially effective in removing surface contamination, but not at re-establishing the oxide layer that provides the corrosion resistance for all stainless steels. Additionally, the bead blasting mentioned may be an additional source of contamination that could require removal. Thanks for the feedback. 

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Determine Passivation of Stainless

Properly cleaned stainless steel is naturally protected from corrosion by a thin, passive film. But this passive layer can be removed or scratched. A passivation meter measures the surface potential under controlled conditions.